The present invention relates to a converting device intended to convert an analog input signal into a digital output signal, having a gain defined as being equal to the ratio between the values of the output and input signals, the device comprising:
a resistor ladder arranged in series between two supply terminals, intended to generate reference voltages, PA1 a plurality of amplifiers, each being intended to deliver an output signal that results from the amplification of a difference between the analog input signal and one of the reference voltages, PA1 a detection stage intended to signal zero-crossings of each of the output signals of said amplifiers, and PA1 a binary encoder intended to deliver the output signal whose value depends on said zero-crossings. PA1 DNL=VDET.GADC/GA, where VDET is an offset voltage introduced by the detection stage, GADC is the gain of the converting device and GA is the gain of the amplifiers. PA1 the adjusting means comprise a first current source arranged in series with the resistor ladder and intended to produce a current whose value is proportional to the value of a control signal, and in that PA1 each amplifier has an adjusting input intended to receive the control signal, while the gain of each amplifier is proportional to the value of the signal received on its adjusting input.
Such devices are known from the article &lt;&lt;An 8-bit Video ADC Incorporating Folding and Interpolation Techniques&gt;&gt;, by Messrs. van de Grift, Rutten and van der Veen, published in IEEE Journal of Solid-State Circuits, vol. SC-22, no. 6, December 1987. These converting devices have a transfer characteristic that describes the evolution of the value of the digital output signal as a function of that of the analog input signal, which transfer characteristic takes the form of a staircase. In an ideal converting device each step of the staircase has a center point whose abscissa corresponds to the value of one of the reference voltages, each center point being in a position to be connected to all the others by the same straight line. It is thus observed that the transfer characteristic of an ideal converting device is linear. In real converting devices, deviations relative to this linearity may occur. For modeling the behavior of the known converting devices and characterizing said deviations, one frequently uses a parameter called "differential non-linearity", defined as being equal to the deviation relative to unity, represented by the least significant bit of the digital output signal, which is shown by the difference between the results of two successive conversions during which the analog input voltage will have taken the value of two successive reference voltages.
An ideal converting device thus has a differential non-linearity equal to zero at any point of its transfer characteristic. If in a real converting device the transfer characteristic shows at a point a non-zero differential non-linearity, said characteristic will show a missing code which is symptomatic of a conversion error.
The gain of the converting devices described above may be expressed in the form of the ratio between the maximum value of the output signal of the converter device and the value of the voltage present on the terminals of the resistor ladder. This gain may be caused to vary, depending on the operating conditions of the converting device. Indeed, it is common practice to make the maximum value of the digital output signal correspond to the maximum value the analog input signal may take, so as to fully use the range of values available for the output signal, which leads to an optimum resolution of the conversion carried out by the device.